If a .410 slug out of a shotgun can kill a deer with a body shot, I have no hesitations about it's effects on a man.
With the buckshot, I might worry about spread and missing the target alltogether, but if they are far enough away for that to happen, then you can probably get off a second shot in time.

If they are far enough away for the shot to spread that far you probably are not in "imminent mortal danger" either. The biggest hinderance I think is the low capacity, and slow cycle rate, not from any lack of power or penetraton.

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"Unfortunately, freedom allows sh*theads to get away with stuff. You always know the mark of a coward. A coward hides behind freedom. A brave person stands in front of freedom and defends it for others."
-Henry Rollins